1. Field of the Invention
This invention relates to low profile additives which control the shrinkage of crosslinkable resin systems when they cure.
2. Introduction to the Invention
Crosslinkable resin systems are well known. It is known that in order to produce such a system which is relatively stable in storage, one of the active chemical moieties (e.g., a catalytic moiety or a reactive moiety) can be present in a xe2x80x9clatentxe2x80x9d form, which can be activated (by heating or otherwise) when rapid reaction is desired. Reference may be made for example to U.S. Pat. Nos. 4,349,651, 4,358,571, 4,420,605, 4.430,445, 4,659,779, 4,689,388, 4,701,378, 4,742,148 and 4,933,392 and European Patent Publication No. 362787A2. Copending, commonly assigned, U.S. application Ser. Nos. 08/726,739, 08/726,740 and 08/726,741 (each of which was filed Oct. 15, 1996 and claims priority from U.S. application Ser. No. 08/399,724 filed Mar. 7, 1995, now abandoned) and corresponding International Application No. PCTJUS96/03023 (published Sep. 12, 1996, as International Publication No. WO-96/27641) disclose particularly valuable latent materials comprising an active chemical moiety which is bonded to a side chain crystalline (SCC) polymer or to another crystalline polymeric moiety which melts over a narrow temperature range. These latent materials, which are referred to in the applications as polymeric modifying agents, are preferably in the form of particles having an average size of 0.1 to 50 microns. Copending, commonly assigned U.S. application Ser. No. 08/710,161 (Docket No. 10762-4 filed Sep. 12, 1996) and corresponding International Application No. PCT/US 97/16019 (which was not published at the date of this application) disclose that even when there is no chemical bond between the active and polymeric moieties, a physical bond between the moieties can produce a lesser but still useful latent effect application Ser. Nos. 08/726,739, 08/726,740, 08/726,741 and 08/710,161 have all been abandoned in favor of a continuation-in-part application, Ser. No. 09/216,520, filed Dec. 16, 1998 U.S. Pat. No. 6,255,367. It is also known that curable resin systems tend to shrink when they cure, and that in some systems this tendency can be lessened or overcome by adding various polymeric additives; such additives are referred to as low profile additives (often abbreviated to xe2x80x9cLPAxe2x80x9ds). Reference may be made for example to pages 48 to 78 (Chapter 4 by Kenneth E. Atkins) in xe2x80x9cSheet Molding Compoundsxe2x80x9d, edited by Hamid Kia (1993), Plastics Compounding, July/August 1988, pages 35-45, and U.S. Pat. Nos. 3,674,893, 3,718,714, 3,721,642, 3,772,241, 3,842,142, 4,125,702, 4,160,759, 4,161,471, 4,245,068, 4,284,736, 4,288,571, 4,374,215, 4,491,642, 4,555,534, 4,673,706, 5,290,854, 5,428,105, 5,504,151, 5,552,478 and 5,589,538.
The disclosure of each of the US patents and patent applications, International and European patent publications, and literature references referred to in the preceding paragraph is incorporated herein by reference for all purposes.
We have discovered, in accordance with the present invention, that the shrinkage that takes place when a resin is cured can be reduced or eliminated by mixing the crosslinkable resin with a novel LPA which is in the form of particles which comprise an SCC polymer (or a similar crystalline polymer which melts over a narrow temperature range). The system is cured at a temperature above the melting point of the crystalline polymer (Tp), and the system and the LPA are such that at least part of the LPA remains as a separate phase in the curable resin at temperatures below Tp. It is believed that at least part of the crystalline polymer remains as a separate phase in the resin as it cures. Preferably at least part of the crystalline polymer forms a separate phase in the cured polymer after it has cooled.
The crystalline polymer can be, but need not be, chemically or physically bound to an active chemical moiety which will take part in the reaction which forms the cured polymer. We believe, therefore, that under appropriate circumstances, some latent materials of the kind described in the copending, commonly assigned US patent applications referred to above will function as LPAs. However, that possibility is not disclosed in those US patent applications or the corresponding PCT applications. Under these circumstances, the extent to which our discovery can be the subject of patent protection may vary from country to country. Accordingly, and since this specification will serve not only as the specification for this US patent application, but also as the priority document for corresponding applications elsewhere, the present invention is broadly defined as any product or process which embodies our discovery and which can properly be the subject of patent protection.
In a first aspect, this invention provides a composition which comprises
(1) a matrix material which
(a) provides a continuous phase, and
(b) comprises at least one precursor which will react to form a crosslinked polymer; and
(2) a low profile additive (LPA) which
(a) comprises a crystalline polymer having an onset of melting temperature To and a peak melting temperature Tp which is (i) at least 30xc2x0 C., and (ii) such that Tpxe2x88x92To is less than Tp0.7, preferably less than 10xc2x0 C., and
(b) is in the form of particles which are uniformly suspended in the matrix material;
said composition, when cured at a temperature of (Tp+10)xc2x0 C. and then cooled to room temperature, having a shrinkage which is less than 0.5 times the shrinkage of a composition which is identical except that it does not contain the LPA and is cured under the same conditions.
The composition may also have at least one of the following characteristics, each of which provides an alternative or additional distinction over the disclosure of the commonly assigned US applications and their PCT equivalents referred to above.
(1) At least 10%, at least 20% or at least 30% of the crystalline polymer is present in the form of particles which do not contain any material which takes part in the reaction which forms the crosslinked polymer.
(2) The composition, when maintained at 40xc2x0 C., doubles in viscosity in less than 240 hours, e.g. in less than 24 hours.
(3) The LPA is present in the form of particles having an average size of at least 75 microns.
(4) When the composition is heated from (Tpxe2x88x9220)xc2x0 C. to (Tp+10)xc2x0 C., there is an increase by a factor of less than 50, e.g., less than 5, in the effective concentration of each of the materials which takes part in the reaction which forms the crosslinked polymer.
It is to be understood that these characteristics (1) to (4) are not intended to represent factors which will be technically advantageous.
The composition may also have one or both of the following characteristics (5) and (6) which will help to reduce the shrinkage of the cured resin.
(5) At most 50% of the crystalline polymer dissolves in the matrix material or in reaction products thereof when the composition is maintained at a temperature of (Tp+10)xc2x0 C. for 15 minutes.
(6) When the composition is cured at a temperature of (Tp+10)xc2x0 C. and then cooled to room temperature, the cured composition contains at least part of the crystalline polymer as a separate phase.
In a second preferred aspect, this invention provides a process for preparing a shaped article of a crosslinked polymer, the process comprising
(A) placing in a mold a composition which comprises
(1) a matrix material which comprises at least one precursor which will react to form a crosslinked polymer, and
(2) a low profile additive (LPA) which is uniformly distributed as a separate phase in the matrix and which comprises a crystalline polymer having an onset of melting temperature To and a peak melting temperature Tp which is (i) at least 30xc2x0 C., and (ii) such that Tpxe2x88x92To is less than Tp0.7;
the composition preferably being placed in the mold at a temperature less than Tp, particularly less than To;
(B) maintaining the composition in the mold at a temperature which is greater than Tp and for a time such that the precursor reacts to form the crosslinked polymer; and
(C) cooling the product of step (B) to obtain the shaped article of the crosslinked polymer;
the composition and the process conditions being such that the shaped article has a shrinkage which is at least one of
(i) a shrinkage of less than 5%, preferably less than 0.5%, particularly less than 0.05%, especially a xe2x80x9cnegative shrinkagexe2x80x9d, i.e. an expansion, and
(ii) a shrinkage which is less than 0.5 times the shrinkage of a shaped article prepared in the same way from a reaction mixture which is the same except that it does not contain the LPA.
The terms xe2x80x9cplacing in a moldxe2x80x9d a composition and xe2x80x9cmaintaining the composition in the moldxe2x80x9d are intended to include any process in which the composition is shaped into a desired shape and is maintained in that shape, or a related shape, while it is being cured. Thus the term includes for example the use of processes in which a closed mold is used and pultrusion processes (in which the composition is cured as it is pulled through a mold).